It is well known that the power and efficiency of gas turbine engines increase with operating temperature. However, the ability of a gas turbine to operate at increasingly higher temperatures is limited by the ability of the turbine components, specifically the vanes, blades, and combustor liners, to withstand the heat, oxidation, and corrosion of the impinging hot gas stream. It is now customary to build turbine components out of nickel-base superalloys which exhibit very good high-temperature mechanical properties and stability. Improved performance is typically obtained by casting the component, e.g., blade or vane, with internal cooling channels that enables the metal to operate at a lower temperature, even when it is in direct contact with the hot combustion gases. Thin ceramic thermal barrier coatings can enhance performance by providing protection against oxidation and corrosion of the hot combustion gas stream. Insulative ceramic coatings can further improve turbine engine performance by reducing the heat transferred into cooled air foils, thereby reducing the requirements for cooling air, which constitutes a performance penalty. The durability of metallic superalloy turbine components is also enhanced by ceramic coatings because they minimize metal temperatures and reduce thermal stresses.
Unfortunately, contemporary thermal barrier coatings, produced by electron-beam evaporation or plasma spraying, have not proven to be very reliable and hence their properties have not been used to full design advantage. Instead, these coatings have been used primarily to extend life and provide a small measure of extra durability.
Contemporary ceramic thermal barrier coatings typically exhibit several layers of differing composition and properties in order to provide a combination of benefits. It is generally preferred to apply a bond coating of an intermetallic such as MCrAlY or PtAl to the metallic substrate before applying a thick, e.g., 125-250 .mu.m ceramic overlay coating. The bond layer is typically preoxidized to form a protective aluminum oxide scale to resist the oxidizing effects of the hot combustion gas stream and to provide an adherent surface for the insulative ceramic thermal barrier. The ceramic thermal barrier coating, TBC, is usually composed of zirconium oxide alloyed with yttria, ceria, magnesia, calcia, or other additives, to help stabilize its composition against undesirable phase transformations, as well as to lower its intrinsic thermal conductivity. Although zirconia is a good insulator, it is highly permeable to oxygen and provides little or no protection against oxidation of the underlying metallic substrate. The passage of oxygen limits the service life of ceramic thermal barrier coating systems at high temperatures due to excessive growth of the aluminum-oxide scale that forms on the bond coating, which leads to the formation of stresses in the interfacial zone, which eventually cause spalling of the insulative layer. These degenerative conditions are enhanced by the severe thermal cycle exposure of the components, which must withstand very hot operating temperatures followed by periods of cooling to near room temperature. When this happens, the thermal expansion mismatch between the metal substrate and the coating introduce additional stresses at the interface, which are superimposed on top of the oxide growth stresses and further accelerate failure.
Consequently, improved, highly durable, ceramic thermal barriers are needed to withstand the rigorous conditions that exist within the combustion path of gas turbine engines. Further, it would be desirable to obtain the maximum performance benefits that thermal barriers offer.
In recent years attention has been directed towards controlling the arrangement of matter at the atomic or molecular scale. Scientists have now demonstrated that nanostructured materials exhibit unusual properties. For example, multilayered nanocomposites consisting of thousands of alternating strata, a few atoms to a few thousand atoms thick, have been shown to have strengths approaching theoretical limits (Steven Ashly, "Small-Scale Structures Yield Property Pay-offs," Mechanical Engineering, No. 52, Feb. 19, 1994). While most of this work has been directed at metallic systems, it has been shown that nanolaminates enable the control of phase composition in zirconia thin film coatings (CR Aita, et al. "Sputter Deposited Zirconia-Alumina Nanolaminate Coatings," J. of Metals, October 1994). In addition, microlaminates of Ce--ZrO.sub.2 /Al.sub.2 O.sub.3 have exhibited three times the toughness of the parent Ce--ZrO.sub.2 material (D. B. Marshall, "Design of High-Toughness Laminar Zirconia Components," Ceramic Bulletin, Vol. 71, No. 6, 1992). In general, both toughness and strength are inversely proportional to layer thickness. Thus, coatings composed of thin nanoscale layers offer the optimum microstructure for structural applications.
While nanostructured materials may offer the potential to improve strength and toughness, it must be shown that thermal properties, e.g., thermal conductivity, are also enhanced and of significant value for thermal barrier applications. It is well established that alloy additions to pure zirconia introduce point defects into the lattice structure which scatter phonons and reduce thermal conductivity (P. J. Klemens, "Thermal Conductivity of Zirconia," Thermal Conductivity, 23, Technomics, Lancaster, Pa., 1996). It has also been shown that a reduction in grain size of yttria stabilized zirconia (YSZ) from one micron to five nanometers (50 angstroms) can theoretically yield a two-fold reduction in thermal conductivity (P. J. Klemens and M. Gell, "Thermal Conductivity of Thermal Barrier Coatings," TBC Workshop sponsored by TBC Interagency Coordinating Committee, 1997).
Multilayered laminates can also be designed in such a way as to reflect radiant energy. When energy enters a transparent medium of one index of refraction and then encounters a transparent medium of a different index of refraction, some of the energy will be transmitted through the discontinuity and some of it will be reflected from it. The reflectance from such an interface, defined as the ratio of the reflected intensity to the incident intensity, is a function of the difference in the indices of refraction between the two individual layers. Thus, an ensemble of thin alternating layers can be designed so that a significant fraction of the input radiant thermal energy will be reflected away. It has also been shown that the turbine blade wall temperature for blades exposed to radiation can be significantly reduced if the ceramic thermal barrier coating is opaque to radiation (R. Siegel and C. M. Spluckler, "Analysis of Thermal Radiation Effects on Temperatures in Turbine Engine Thermal Barrier Coatings," TBC Workshop, Cincinnati, Ohio 1997).
Further improvements are needed to extend the useful life of thermal barrier coating systems and enable them to survive the increasingly severe operating conditions needed to obtain higher performance in gas turbine engines. It would be highly desirable to employ multiple nanoscale ceramic layers to scatter and reflect thermal energy and thereby enhance the performance of gas turbine engine components.